Identifying intellectual disability: Some problems in the measurement of intelligence and adaptive behaviour

Abstract

Definitions of intellectual disability have traditionally depended on psychometric conceptions of intelligence and the use, with varying degrees of flexibility, of cut‐off scores on intelligence tests. More recently, deficits in adaptive behaviour as well as in intelligence have been incorporated into definitions of intellectual disability in Australia as well as overseas, and identification is directed towards establishing support needs rather than identifying group or category membership. This paper explores some of the difficulties inherent in the use of psychometric concepts and measurements to identify intellectual disability from a needs perspective. Problems of test reliability and the implications of changes over time in intelligence test norms are discussed. The concept of adaptive behaviour, its construct validity, and the utility of a psychometric definition of adaptive behaviour are questioned. It is concluded that a more fruitful definition of intellectual disability might be found in psychological theories about human needs.